Spatial tracking systems allow applications to capture data about an object's position or a characteristic of position. Position can be one or more characteristics of position and can be relative, absolute, or a combination of both, depending on the spatial tracking system. Relative position is the position of the object in relation to an initial location, and may be determined based on resolving motion of the object in terms of directional vectors. Dead reckoning systems use relative position to determine spatial information about an object. Absolute position is the position of the object within the spatial tracking system, and may be determined without prior knowledge of where the object has been. Accordingly, absolute position is not dependent on knowledge of an initial location or state.
In many applications, spatial tracking systems provide multi-axis information to track position and rotation about three axes. This type of tracking system may be called a six-axis tracking system. For example, a two-axis spatial tracking system with position and rotation sensors may determine the position of an object along an axis, such as +5 units along the X-axis. Further, that object may have a rotational angle about the axis, such as 45° about the X-axis. This two-axis spatial tracking system may be further expanded to determine position and rotation in three axes, such as the X-axis, Y-axis, and Z-axis, to yield a six-axis tracking system. An example coordinate system that may facilitate determining rotation and position is shown in FIG. 3 with X, Y, and Z axes and corresponding θx, θy, and θz rotational angles. The corresponding θx, θy, and θz rotational angles are also known respectively as pitch, roll, and yaw.
Spatial tracking systems capable of determining position and rotation of an object in space have been used in hosts of applications, including computers, video game systems, remote controlled devices, and robots. Other applications with spatial tracking systems include joysticks, computer mice, or video game controller. In many of these applications, a portable device determines position and rotation of itself and provides this information to an external device, such as a personal computer, for further processing. In other applications, the external device may determine position and rotation of the portable device based on raw sensor information relayed from the portable device.
Many solutions for determining position and rotation of a portable device use sensors located within the portable device. For example, portable devices may include sensors such as at least one of a potentiometer, a Hall effect sensor, a rotary encoder, a camera, an infrared sensor, a gyroscope, and an accelerometer to determine position and rotation. However, many portable devices using these sensors exhibit user interface problems related to controlling the portable device in a simple manner. For example, a computer mouse used to control multiple axes may use certain click and drag combinations to inform the external device which axis to perform a motion in. These types of complex movements may not be intuitive to the user, which detracts from the portable device's usability.
Spatial tracking systems that use cameras or infrared sensors often experience similar usability drawbacks because of environmental factors. Cameras and infrared sensors may have difficulty producing viable information when not placed in a controlled environment. A camera or infrared sensor may be susceptible to ambient light changes such that sensor output may be distorted or unusable for processing by the external device. Additionally, the camera or infrared sensor may cease to function if used with an unsuitable surface or if the sensor becomes blocked.
Accelerometers also have been used to determine position and rotation in spatial tracking systems. An accelerometer can measure instantaneous acceleration along an axis, which if integrated twice yields displacement or relative position. Put another way, accelerometers do not measure position directly, but can determine displacement through integration of acceleration data. Displacement is the movement of an object relative to an initial position. If three accelerometers that are orthogonal to each other are used to determine position, then displacement in three-dimensional space may be determined using their outputs. A spatial tracking system may also use the effect of gravity on the accelerometers to determine rotation about an axis.
Configurations using accelerometers also have limitations. Integrating acceleration information twice to produce a position may introduce errors that cause the spatial tracking system to lose accuracy quickly. The first integration operation of acceleration to velocity may introduce some amount of error. The second integration operation of velocity to position may integrate this error to introduce even more error. These errors may result in the system determining that the portable device is moving even though the portable device is at rest.
This effect is known as drift. As a result, algorithms and other sensors, such as gyroscopes, have been used in combination with accelerometers to produce a spatial tracking system capable of determining position and rotation. However, these additional sensors may also be prone to drift, and so even more additional sensors, such as a magnetometers, may be used to improve accuracy of the spatial tracking system. These additional sensors can increase the cost and complexity of the portable device.
In addition to spatial tracking systems discussed above, wireless power systems are known for transferring power wirelessly in hosts of applications, including wireless power transfer to portable devices. Wireless power supply systems enable power to be transferred without direct electrical connections using inductive coupling between inductors or coils. Many applications have used this capability to transfer power wirelessly. The prior art shown in FIGS. 1 and 2 shows one example of a wireless power system in which a portable device may receive power from a primary coil using three orthogonal secondary coils. The wireless power supply may be designed to drive the primary coil to achieve efficient wireless power transfer between the wireless power supply and the portable device. This wireless power system may be configured to allow the portable device to receive power from the primary coil via inductive coupling despite the orientation of the portable device relative to the primary coil. This wireless power system, however, is not configured to determine position or rotation of the portable device.
Additionally, wireless power systems can affect performance of sensors used for determining position and rotation. The magnetic field used for inductive coupling between the wireless power supply and the portable device(s) may prevent some sensors from producing an accurate output. For example, a magnetometer used to measure the earth's magnetic field may cease to function inside the magnetic field of the wireless power system. Accordingly, complex and expensive sensor systems may be used to both transfer power wirelessly to the portable device and determine position and rotation of the portable device.